Traditionally, a sheet made of a nonwoven fabric has been used in many cases as an air filter material for removing fine dust such as pollen or coarse particulates. Such an air filter material is required to have performance for collecting dust with a high efficiency, and a low pressure loss by which inhalation resistance is low when a gas passes through the air filter material. The air filter material allows collection of the fine dust such as the pollen and the coarse particulates by an electrostatic attraction and so forth in addition to a physical collection action by using a compacted matrix formed of fibers having a small fiber diameter or a filter subjected to electret processing.
A melt-blown nonwoven fabric including a mean fiber diameter of 15 micrometers or less frequently is used for the air filter material in order to form the compacted matrix. As a shape to be used, a product processed into a cup shape or a pleated form is used.
On the other hand, the melt-blown nonwoven fabric includes thin fibers and is subsequently flexible. Therefore, a packing material or a reinforcement has been needed when the melt-blown nonwoven fabric is used as the air filter material. Patent document No. 1 has proposed an air filter including biodegradable fibers in which a melt-blown nonwoven fabric and a spunbond nonwoven fabric are laminated. The filter material has a disadvantage of an increased pressure loss because a layer of the melt-blown nonwoven fabric and a layer of the spunbond nonwoven fabric are bonded by a number of partially fused parts by using an embossing roll.
Patent document No. 2 has proposed a laminated filter including a plurality of synthetic fibrous sheets having a different melting start temperature. A material of the laminated filter has a disadvantage of a low collection efficiency because the material is a combination of a melt-blown nonwoven fabric subjected to electret processing and a spunbond nonwoven fabric, and is not subjected to electret processing on a side of the spunbond nonwoven fabric.